In the drilling of deep boreholes such as oil wells it is desirable to measure certain downhole parameters and to telemeter the information to the surface of the earth. Many efforts have been made to develop a successful telemetering system for performing logging while drilling operations.
For instance in the disclosure of the U.S. Pat. No. 2,096,279 issued to Karcher, a system has been proposed for utilizing electrical conductors inside a drill pipe. Another system has been proposed in the disclosure of the U.S. Pat. No. 2,354,887 issued to Silverman, wherein inductive coupling of a coil or coils with a drill pipe was utilized for transmitting downhole information to the earth's surface. A different system has been described in the U.S. Pat. No. 4,001,773 issued to Lamel, wherein torsional waves travelling along a pipe have been used to convey the useful information.
Perhaps the most promising of these prior art suggestions in a practical sense has been that of signalling by pressure pulses in the drilling fluid. Signalling by pressure pulses can be achieved by means of positive or negative pressure pulses. A positive pressure pulse generator has been described, for instance, in the U.S. Pat. No. 2,787,759 issued to Arps and in the U.S. Pat. No. 3,736,558 issued to Cubberly, Jr. A negative pressure pulse generator has been described, for instance, in the U.S. Pat. No. 3,964,556 issued to Gearhart et al, and in the British Patent Application No. 2,009,473A issued to Scherbatskoy and published June 13, 1979. Positive-pulse systems use a downhole valve which restricts mud flow to the bit to produce a pressure increase or a positive pulse. Negative pulse systems bypass the pressure drop across the bit, thus producing a pressure decrease or a negative pulse.
In various attempts in the prior art to develop a successful telemetering system, a serious problem was caused by the presence of a high level of ambient noise. It has been found difficult, if not impossible, to extract from measurements performed while drilling the useful downhole information. The useful information carrying signal is generally obscured by noise accompanying normal drilling operations. The noise is generated by the cutting of the drill bit into the formations, by the operation of the pump which forces drilling fluid into the circulation system and by various other extraneous causes. This noise is of a magnitude sufficient to seriously interfere with the proper detection of the signal.
A general objective of my invention is to provide a method and apparatus for reducing, as much as possible, the effects of noise generated during normal drilling operations so as to improve the performance of measurements while drilling systems such as those described in the disclosures of U.S. Patents cited above.
Another objective of my invention relates specifically to those telemetering systems which utilize positive or negative pressure pulses in the drilling fluid for transmitting downhole information to the earth's surface. In such systems the main source of noise results from the operation of a pump.
Thus the U.S. Pat. No. 3,302,457 issued to Mayes describes a system for achieving pump stroke elimination by means of a differential pressure transducer which senses a pressure drop across a section of drilling flow path and feeds a current proportional to this differential pressure into an anlog network which represents the drilling fluid flow path. There is an obvious disadvantage in the system proposed by Mayes. It is extremely difficult, if not impossible, to design the required analog network which would provide a reasonable approximation to the drilling flow system in a borehole.
A different approach to the problem of pump strke attenuation has been suggested in the disclosures of the patents as follows: U.S. Pat. No. 3,488,629 issued to Claycomb, U.S. Pat. No. 3,555,504 issued to Fields, U.S. Pat. No. 3,716,830 issued to Garcia, and U.S. Pat. No. 3,747,059 issued to Garcia. In these disclosures a directionally discriminating system has been proposed for effectively cancelling the pressure pulse variations due to the mud pump and for permitting downhole pressure pulse signals from a measuring instrument to be detected at the surface. However, it should be noted that such a directional discrimination does not provide an adequate selectivity which would insure a satisfactory signal to noise ratio for a measurement while drilling system.
Another arrangement for pump stroke attenuation has been suggested by Claycomb in the U.S. Pat. No. 4,262,343.
A different approach for attenuating interfering effects due to pump strokes has been suggested by Foster, et al in the U.S. Pat. No. 3,742,443. In an arrangement described in this patent two transducers are used at spaced points in the fluid circulation system for producing signals representing variations in fluid pressure at these points. These signals are subsequently processed and combined. The processing requires two filters. One of these is a time delay filter and the other is a distortion compensating filter. The design of these filters depends on the transmission characteristics of the fluid circulation system between the two spaced points at which the transducers are located. Such transmission characteristics are difficult to obtain. They would require special measuring equipment and a specialized personnel in addition to the personnel generally used for supervising drilling operations. The arrangement suggested by Foster, et al is cumbersome and impractical in operation.
One of the features of this invention which distinguishes it from the prior art is using an indication representing the interfering effects for enhancement of the signal to noise ratio.
In accordance with another feature of this invention, the recurrence in the pump operation is directly used to develop a subtraction procedure which is effective to improve the signal to noise ratio at the receiving end of the telemetry system.
Other methods and objectives of this invention will be apparent when making reference to the attached claims.